kbuild: consolidate version and timestamp headers generation

- Generate include/generated/{timestamp.h, version.h}
more simply by using filechk rule.
- Add $(UBOOTRELEASE) variable and re-write u-boot.imx rule
more simply.
- Rename U_BOOT_VERSION in Makefile to UBOOTVERSION
Before this commit, the same variable name, "U_BOOT_VERSION"
was used for two different strings.
One of them was defined in Makefile.
It takes the form like "2014.01-rc1" and used in
makefiles and script files.
The other is defined in include/generated/version.h
It takes the form like "U-Boot 2014.01-rc1-00010-gbe6d4266-dirty"
and used in C and Aseembler.
It is confusing when grepping the source tree. So, this commit
renames the former to UBOOTVERSION.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>